How To Choose The Right Engine For Your Boat

How to choose the right engine for your boat.

What's the best engine for your boat? To answer this age-old question, we arranged with Steve Fleming at Mercury Marine and Chuck Thompson at Trophy to borrow three identical 2003 22-ft. Trophy Walkarounds, each powered by a different Mercury Marine engine/drive setup of roughly 135 hp and 200 ft.-lb. of torque.

Trophy's standard setup for its 2052 Walkaround is the MerCruiser 3.0L. This is an overhead-valve inline Four of 3.0 liters and 135 hp, with two valves per cylinder and a 2-barrel carburetor. It runs happily on 87 octane gasoline. Connected to MerCruiser's equally basic Alpha 1 sterndrive, the engine/drive unit weighs 635 pounds and costs just $6899.

With 85 gal. of gasoline in the tanks, two men and some test gear, our Trophy weighed just over 4400 pounds, ready to go. Thanks to judicious use of the hydraulic trim tabs, we accelerated from idle to 30 mph in 13.5 seconds and repeatedly topped out at precisely 34.7 mph.

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At wide-open throttle (WOT) the engine was turning 4800 rpm and gulping down 11.9 gal. of gas per hour (gph). That's 2.92 mpg. At a more relaxed speed of 20 mph, it returned 3.23 mpg to give a cruising range of 274 miles. In our neighborhood, a gallon of 87 octane currently sells for $1.79. We calculate a fuel cost of about 56 cents per mile at 20 mph or about 62 cents per mile at 34.7 mph.

Mercury's 135-hp Optimax has the same power rating as the 3.0L sterndrive. A 2-cycle V6 of 2.5 liters, it's fed by Mercury's 2-stage direct fuel injection and runs on 87 octane gasoline. The MSRP on the outboard is $10,615. The basic outboard weighs 431 pounds, a significant 200 pounds less than the 3.0L sterndrive. However, the 2-stroke outboard requires a separate 3-gal. oil tank, different rigging and a reinforced transom. Bottom line, the dry weight for the 2002 Walkaround outboard is virtually identical to that of the 2052 Walkaround sterndrive.

The outboard and sterndrive may both be rated at 135 hp, but the performance is markedly different. The gas sterndrive out-accelerates the Optimax outboard at speeds up to 20 mph. Between 20 and 25 mph, however, the lines on the graph cross and the outboard handily outruns the sterndrive. The 2002 Walkaround is 2.4 seconds quicker to 30 mph and tops out at 42.5 mph. That's nearly 8 mph faster than the sterndrive.

You pay for this performance. At WOT, the Optimax burns 14.2 gph. At the 34.7-mph top speed of the sterndrive, the outboard burns 13.4 gph. Throttle back to a 20-mph cruise, however, and the outboard delivers a comparable 6.5 gph. We calculate a cruising range of 261 miles for the Trophy 2002 and a fuel cost of roughly 58 cents per mile at 20 mph. At 34.7 mph, the outboard fuel costs around 69 cents per mile.

But wait. A 2-cycle engine lubricates its bearings by oil injection. Even something as clean-running as this Optimax 135 still goes through roughly 1.7 gal. of oil for every 85-gal. tank of gas. In our neighborhood, Mercury Marine's Quicksilver 2-Cycle Outboard Oil sells for $15.99 per gallon. This calculates to about 12 cents per mile. We get a total cost for fuel and oil of about 71 cents per mile at 20 mph and 82 cents per mile at 34.7 mph.

Cummins and MerCruiser have a new joint venture building small diesels for pleasure boats. We tried a 1.7-liter turbocharged inline Four rated at 120 hp and 195 ft.-lb. of torque. Equipped with the same Alpha 1 lower unit as the MerCruiser 3.0L gas sterndrive, the engine/drive package costs $12,000.

The diesel is slower in acceleration than the gas sterndrive and outboard in every speed range. Surprisingly, the diesel makes it to the same top speed as the 3.0L sterndrive.

The diesel is significantly quieter than the 3.0L sterndrive at every speed, and is even quieter than the Optimax outboard. The diesel is also more fuel efficient. At a 20-mph cruising speed, the diesel sips just 3.3 gph (6.06 mpg). In this Trophy Walkaround, that's a cruising range of 515 miles at 20 mph. Even running at WOT, the diesel consumes only 5.8 gph at 34.7 mph (5.98 mpg).

In our neighborhood, diesel fuel is now $1.99, 20 cents more than 87 octane unleaded. At 20 mph, the Cummins/MerCruiser diesel costs about 33 cents per mile to run. At 34.7 mph, it still costs just 34 cents per mile.

Payback
A 135-hp 3.0L sterndrive costs $3716 less than a 135-hp Optimax outboard. At 34.7 mph, it also saves 20 cents per mile in fuel and oil. And it's quieter. The outboard's only advantages are slightly quicker acceleration and a higher top speed.

The 1.7 MS120 diesel is quieter, slower in acceleration, and costs $1385 more than the Optimax and $5101 more than the sterndrive. However, it burns significantly less fuel, compared to the Optimax, and will offset the extra cost in just 2924 miles at 34.7 mph.

The diesel sips half as much fuel as the gas sterndrive, but, at 20 mph, it will take you 22,373 miles to recover the extra initial cost of the diesel engine. At 34.7 mph, it will take 18,719 miles to break even. That's 540 hours of running, which for the average American boater is five summers worth of cruising the lake.

Still, we know which powerplant we'd choose. MerCruiser's brilliant new turbodiesel is quieter, cheaper to run, and offers nearly twice the cruising range of either gas engine. It would lure us out on the water to rack up those pleasure-filled miles till the diesel paid for itself many times over.